TPS62240, TPS62242, TPS62243

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SLVS762B–JUNE 2007–REVISED SEPTEMBER 2007

DETAILED DESCRIPTION

OPERATION

The TPS62240 step down converter operates with typically 2.25MHz fixed frequency pulse width modulation (PWM) at moderate to heavy load currents. At light load currents, the converter can automatically enter Power Save Mode and operates then in PFM mode.

During PWM operation, the converter uses a unique fast response voltage mode control scheme with input voltage feed-forward to achieve good line and load regulation, allowing the use of small ceramic input and output capacitors. At the beginning of each clock cycle initiated by the clock signal, the High Side MOSFET switch is turned on. The current then flows from the input capacitor via the High Side MOSFET switch through the inductor to the output capacitor and load. During this phase, the current ramps up until the PWM comparator trips and the control logic turns off the switch. The current limit comparator also turns off the switch if the current limit of the High Side MOSFET switch is exceeded. After a dead time preventing shoot through current, the Low Side MOSFET rectifier is turned on and the inductor current ramps down. The current then flows from the inductor to the output capacitor and to the load. It returns back to the inductor through the Low Side MOSFET rectifier.

The next cycle is initiated by the clock signal again turning off the Low Side MOSFET rectifier and turning on the High Side MOSFET switch.

POWER SAVE MODE

The Power Save Mode is enabled with MODE Pin set to low level. If the load current decreases, the converter will enter Power Save Mode operation automatically. During Power Save Mode, the converter skips switching and operates with reduced frequency in PFM mode with a minimum quiescent current to maintain high efficiency.

The transition from PWM mode to PFM mode occurs once the inductor current in the Low Side MOSFET switch becomes zero, which indicates discontinuous conduction mode.

During the Power Save Mode, the output voltage is monitored with a PFM comparator. As the output voltage falls below the PFM comparator threshold of VOUT nominal, the device starts a PFM current pulse. The High Side MOSFET switch will turn on, and the inductor current ramps up. After the On-time expires, the switch is turned off and the Low Side MOSFET switch is turned on until the inductor current becomes zero.

The converter effectively delivers a current to the output capacitor and the load. If the load is below the delivered current, the output voltage will rise. If the output voltage is equal to or greater than the PFM comparator threshold, the device stops switching and enters a sleep mode with typical 15-μA current consumption.

If the output voltage is still below the PFM comparator threshold, a sequence of further PFM current pulses are generated until the PFM comparator threshold is reached. The converter starts switching again once the output voltage drops below the PFM comparator threshold.

With a fast single-threshold comparator, the output voltage ripple during PFM mode operation can be kept to a minimum. The PFM Pulse is time controlled, allowing the user to modify the charge transferred to the output capacitor by the value of the inductor. The resulting PFM output voltage ripple and PFM frequency both depend on the size of the output capacitor and the inductor value. Increasing output capacitor values and inductor values will minimize the output ripple. The PFM frequency decreases with smaller inductor values and increases with larger values.

If the output current cannot be supported in PFM mode, the device exits PFM mode and enters PWM mode. The Power Save Mode can be disabled through the MODE pin set to high. The converter will then operate in fixed frequency PWM mode.

Product Folder Link(s): TPS62240 TPS62242 TPS62243